The present invention relates to an anchoring system for a surface installation subject to high frequency movements.
One specific proposed application is to marine loading buoys for transferring hydrocarbons, these buoys being adapted to float on the surface of the water.
These marine buoys are positioned in the vicinity of an oil platform which is installed above an undersea oil field. The marine buoys are then connected to the platform and the hydrocarbon is transferred from the platform to the buoys and passed from the buoys to a surface vessel moored to the buoy.
This marine loading buoy is obviously moored to the seabed by at least one anchor line extending between the buoy and the bed. This anchor line generally comprises a chain of which one end is anchored to the seabed and the other end is attached to the marine loading buoy by means of an attachment device. In some cases, these anchor lines are composite, having a central part composed of a cable, and two ends each terminating in a chain.
Although the marine loading buoy is generally moored with a plurality of anchor lines which are relatively tightly stretched, it is moved by the wind, the swell and/or the surface currents, in such a way that severe fatigue stress is exerted on these anchor lines and particularly on the links of the chains. This is because, unlike a ship or a platform which is relatively stable with respect to these surface movements because of its inertia, a marine loading buoy, being lighter, is constantly moved at high frequencies with respect to the anchor lines which retain it.
Thus, in order to attach the chain to the marine installation, anchor lines generally comprise an attachment device which is pivoted about two pivot pins. This attachment device has at least two flanges mounted with one pivoting on the other, one of the flanges being mounted rotatably in a shackle which is fixed to the marine installation, while the other flange is fixed to the chain.
This is because the frictional forces arising in the pivots create a resistive torque which is proportional to the coefficients of friction, to the tension of the chain and to the diameter of the pivot axis. To enable pivoting to take place, this resistive torque must be overcome by a torque created by the angle of deflection of the end of the chain with respect to the end of the flange. Although the links of the chains are capable of pivoting with respect to each other, it is found that the chain becomes virtually rigid because of the high degree of tension, and this torque can only be generated by the bending of the links. Although the angles of deflection are less than 1°, this bending still causes fatigue in the chain which can adversely affect its service life.
Above this angle, the torque is sufficient to cause the attachment device to pivot.